Properly Document how to override core classes

[weaverryan]

Hi guys!

First, see symfony/symfony#11881. Basically, we should be using service decoration and not recommending that you override any *class parameters. We do talk about service decoration in components/dependency_injection/advanced.rst, but I propose that we show a quick cookbook article on how to override a real core service.

It might also be nice to change the existing article to use examples better than foo and bar :). See https://github.com/symfony/symfony-docs/pull/3753/files#r17416550.

[wouterj]

Please note that this duplicates what I proposed in #4228:

Three topics ("Marking services as public/private", "Aliasing" and "Decorating Services") are related to the way you make your services open for extensibility. This should be moved into a new article, telling you everything about these concepts and learning you best practises.

[linaori]

@weaverryan Is it possible to put this in the 2.6 milestone? Especially with new services not having .class, this will be a must have for the documentation.

[wouterj]

@iltar that's not how we use the milestones. Milestones in de docs indicate only new features which needs to be documented for a specific version. This should be done for all versions.

It's still an issue that's high on my prioritylist btw.

[weaverryan]

I agree this should be done soon, and me can merge it into all branches, which is really nice :).

Cheers!

[linaori]

I've got a service here that might want some love and attention. A lot of people need the following service but a bit more customized, without copying all the code to a SimpleAuthenticatorInterface implementation. I often see question related to this in #symfony. There are a few more but I can't remember them, if I find more example worthy services, I'll let you know.

security.authentication.provider.dao - Using a decorator around DaoAuthenticationProvider which implements SimpleAuthenticatorInterface to allow it in the SimpleAuthenticationProvider without having to duplicate too much code.

[wouterj]

Closing in favor of #4228